The present invention relates, in general, to implantable medical devices, and, in particular, to new and useful bioabsorbable medical devices that are capable of being a self-regulating system for controlling the acidic effects of degradation. Additionally, the present invention relates, in particular, to bioabsorbable medical devices for vascular or cardiovascular applications that can control the acidic effects of degradation.
Bioabsorbable implants are typically made from polymeric materials such as lactone-based polyesters. These bulk eroding materials breakdown over time due to chemical hydrolysis to produce water-soluble, low molecular weight fragments. These fragments are then attacked by enzymes to produce lower molecular weight metabolites. Acid fragments that are produced during degradation of the polymer backbone have shown to cause local tissue inflammation. The inflammation has been observed in vascular systems as well and the extent of inflammation depends on the pH of the acid that in turn is dependent on the type and amount of acid produced during degradation. This inflammation is not typically observed in polymers that degrade by surface erosion (such as polyorthoesters and polyanhydrides) as the amount of acid released at a given time is small to cause tissue inflammation.
Additionally, most of the past research in the field of bioabsorbable implants has been directed toward orthopedic applications, for instance, toward using a bioabsorbable implant as internal fixation devices in bone. Thus, this trend is specifically toward internal fixation devices for repair of damaged bone through the use of resorbable, tissue compatible biopolymers. Biopolymers such as poly(glycolic acid) [PGA], poly(lactide) [PLA], and copolymers of lactic and glycolic acids, [poly(lactide-co-glycolide) or PLGA] have been used in the production of internal fixation devices, such as screws, pins, and rods to hold bone together following surgery, or to repair broken bones. Other polymers, such as poly(dioxanone), have also been considered for use in the manufacture of surgical internal fixation devices. However, it has been observed that tissue response to resorbable implants fabricated from these biopolymers is not uniformly acceptable (Bostman, J. Bone and Joint Surg. 73, 148-153 (1991).
The tissue response to these biopolymer-based orthopedic implants has been well documented. Late sterile inflammatory foreign body response (sterile abscess) has been reported in about 8% of fractures repaired with these polymers (Bostman, supra). In a randomized study of 56 open reduction and internal fixation of malleolar fractures of the ankle with metal ASIF screws and plates or with rods of PLGA, two cases of sterile inflammatory wound sinus were observed 3 to 4 months after the operation in the injuries fixed with the polymer rods (Rokkanen et al., Lancet 1, 1422-1425 (1985); Bostman et al., J. Bone and Joint Surg., 69-B(4), 615-619 (1987)).
Other orthopedic studies have also documented an inflammatory reaction following implantation of PGA or PLGA orthopedic fixation devices. The fraction of patients suffering from this reaction ranges from 4.6 to 22.5% (Bostman et al., Clin. Orthop. 238, 195-203 (1989); Bostman et al., Internat. Orthop. 14, 1-8 (1990); Hirvensalo et al., Acta Orthop. Scandinavica, Supplementum 227, 78-79 (1988); Hoffman et al., Unfallchirurgie 92, 430-434 (1989); Partio et al., Acta Orthop. Scandinavica, Supplementum 237, 43-44 (1990); Bostman et al., Internat. Orthop. 14, 1-8 (1990)).
Moreover, the inflammatory reaction is not limited to orthopedic implants made from poly(glycolide) polymers. Internal fixation devices made from poly(lactide) have also been observed to exhibit an inflammatory reaction. Eitenmuller et al. reports that 9 of 19 patients (47.7%) who had fractures of the ankle treated with absorbable plates and screws of poly(lactide) had an inflammatory response. (J. Eitenmuller, A. David, A. Pomoner, and G. Muhyr: “Die Versorgung von Sprunggelenlzsfrakturen unter Verwendung von Platten und Schrauben aus resorbserbarem Polymermaterial”, Read at Jahrestagung der Deutschen Gesellschaft fur Unfallheilkunde, Berlin, Nov. 22, 1989).
Additionally, in vitro studies have been performed to monitor pH changes as well as weight loss and the appearance of lactic acid from orthopedic screws fabricated from poly(lactide-co-glycolide) with a lactide:glycolide ratio of 85:15. (Vert et al., J. Controlled Release 16, 15-26 (1991)). An induction period of about ten weeks was observed before any significant change in media pH or weight loss occurred. This time period corresponds to the induction periods of seven to twenty weeks noted by orthopedic clinicians. However, no attempt had been made to alleviate the source of inflammation.
One known in vitro study involving orthopedic implants is described in J Biomed Mater Res (Appl Biomater) 38: 105-114, 1997 and was performed to examine if the pH decrease in the vicinity of degrading polylactic acid (PLA) and polyglycolic acid (PGA) polymers could be offset by incorporation of basic salts within PLA-PGA orthopedic implants. It had been suggested that such pH lowering results in adverse effects, which may be responsible for biocompatibility concerns raised recently about PLA and PGA polymers. Accordingly, this study was conducted and the results indicated that all three salts investigated in this study were successful in controlling the decrease in pH due to the acidic degradation products of the copolymer. The pH of the test media for the control group fell to a value of 3.0 at 9 weeks. Implants containing calcium carbonate maintained the pH value between 7.4 and 6.3 throughout the degradation process. Implants with calcium hydroxyapatite and sodium bicarbonate controlled the pH values between 6.9 and 4.3 and 8.2 and 4.5, respectively. At 3 weeks, marked swelling of implants containing calcium carbonate or sodium bicarbonate was observed relative to the control orthopedic implants. The molecular weight and mass changes in the orthopedic implants did not show any significant differences at 9 weeks. Thus, results from this in vitro study showed that a significant decrease in pH in the vicinity of a PLA-PGA orthopedic implant could be avoided by incorporating basic salts into the orthopedic implant itself.
To date, there have been no known bioabsorbable medical devices that are capable of being a self-regulating system for controlling the acidic effects of degradation. Additionally, to date, there have been no known bioabsorbable medical devices for vascular or cardiovascular applications that can control the acidic effects of degradation.